1. Field of the Invention
The present invention relates to membranes having molecular sieve properties and/or catalytic activity and to methods for producing and using the membranes, and more particularly, to isomorphously substituted zeolite membranes and their use in selective separations of molecules and in catalytic membrane reactors.
2. Discussion
Zeolites are crystalline aluminosilicates of Group 1 and Group 2 elements. Their basic structural framework can be viewed as a three-dimensional network of SiO4 and [AlO4]− tetrahedra, which are linked by oxygen atoms. The structural framework encloses cavities and defines channels or pores that are substantially uniform in size within a specific zeolite. As discussed below, large ions (compensating cations, M) and water molecules occupy some of the cavities and have considerable freedom of movement within the zeolite lattice, which allows zeolites to perform ion exchange processes and reversible dehydration.
Because the zeolite pores are sized to accept molecules of certain dimensions for adsorption while rejecting molecules of larger dimensions, these molecules have come to be known as “molecular sieves.” Zeolites have been used commercially in ways that that take advantage of these properties, including adsorption separation processes and shape-selective catalytic processes.
Most commercial applications use zeolites in the form of granules or pellets. Zeolite granules exhibit high porosity and have a uniform pore size between about 0.3 and 1.2 nm that is dependent on the specific zeolite structure. Such granules are the catalysts of choice for the petrochemical industry. Shape-selective effects are possible because the catalytic sites are accessible only within the pores of a zeolite structure, and only those reactant molecules, transition states, intermediates, and/or product molecules with dimensions below a certain critical size can be adsorbed into this pore system. Shape-selective catalysis combines the molecular sieving effect with a catalyzed reaction.
Recently, zeolite membranes have been used to conduct molecular separations. Generally, a membrane can be defined as a semi-permeable barrier between two phases that is capable of restricting the movement of molecules across it in a very specific manner. The semi-permeable nature of the barrier is essential to obtaining an effective separation. A wide variety of molecular materials, mostly organic polymers, have been found to be suitable for use as membranes. However, organic polymer membranes have relatively short service lives because of their sensitivity to solvents and low stability at high temperatures.
Because of their superior thermal, chemical, and mechanical properties, zeolite membranes have substantial advantages over organic polymer membranes. The pore size is uniform within a specific zeolite material, and the pore size of a zeolite membrane can be synthetically tuned by choosing an appropriate zeolite structure and/or by exchanging compensating cations of different diameters. The hydrophilic/hydrophobic nature of a zeolite can be modified by changing the substituted metal (Me) in the framework and the Si/Me ratio. The basic/acidic nature of the zeolite can be modified by exchanging alkaline cations with protons. Moreover, zeolite membranes can be used for catalytic membrane reactors because they combine heterogeneous catalytic sites with membranes that allow only one component of a mixture to selectively permeate across the membrane.
Zeolites can be considered as originating from a SiO2 lattice in which Al3+ is isomorphously substituted for a portion of tetrahedrally coordinated Si4+, and can be represented by the formula:Mx/n·[(AlO2)x·(SiO2)y]·zH2O  Iwhere M represents a compensating cation with valence n, y is a number greater than or equal to x, and z is a number between about 10 and 10,000. In an isomorphous substitution, a second (different) element replaces some (or all) of an original element of the crystalline lattice. The second element has similar cation radius and coordination requirements as the original element so that the same basic crystalline structure is maintained.
Because aluminum is trivalent, every tetrahedral [AlO4] unit carries a negative charge. Consequently, the substitution of aluminum for silicon generates an excess negative charge in the zeolite lattice that must be compensated by cations. These compensating cations may be exchangeable. Accordingly, the ion-exchange capacity of a zeolite is enhanced as the aluminum content is increased. Acid hydrogen forms of zeolites have protons that are loosely attached to their framework structure in lieu of inorganic compensating cations, and these proton sites function as Brönsted acids. Thus, the number of protons that may be attached to the zeolite framework is greater in zeolites having greater aluminum content. Consequently, increases in the aluminum content of a zeolite can result in additional Brönsted acid sites. Zeolites having additional catalytic sites exhibit greater activity in acid catalyzed reactions. Thus, the ion exchange and the catalytic properties of a specific zeolite depend on its chemical composition and, more particularly, on its Si/Al ratio.
Zeolites represented by formula I are often described in terms of their Si/Al ratio, because certain properties of zeolites appear to vary with Si/Al ratio. In an extreme case in which substantially all of the lattice ions are silicon, zeolites can have Si/Al ratios that approach infinity (e.g., silicalite-1). Such zeolites do not have a net negative framework charge and therefore do not contain compensating cations. As a consequence, these zeolites have no ion exchange capacity, cannot be acidic, and exhibit a high degree of hydrophobicity. These highly siliceous zeolites are organophilic and have been used for the selective adsorption of volatile organic compounds. Zeolites with Si/Al ratios as low as 0.5 have also been made (e.g. bicchulite).
With zeolite membranes, separation is thought to occur through at least three different, nonexclusive mechanisms, which are based on differences in component diffusion, on molecular sieving or size exclusion, and on preferential adsorption. Thus, two or more different types of molecules may access the pore system of the zeolite membrane, but their diffusion rates through the pores may vary because each type of molecule interacts differently with the zeolite surface and pore structure. Additionally, molecular sieving may occur when one type of molecule can access the zeolite membrane pore system, but a different type of molecule cannot because of its larger size. Finally, the pore system of the zeolite membrane may preferentially adsorb a first molecule, which blocks entry of a second, different molecule into the pore system. Because molecules with different sizes and shapes have different diffusivities, high separation selectivities have been reported for n-C4H10/i-C4H10, and n-C6H14/3-methyl pentane mixtures. Likewise, high separation selectivities based on molecular sieving were obtained for CH4/i-C8, n-C6/2,2 dimethylbutane, and p-/o-xylene mixtures. Selectivities have also been attributed to differences in adsorption properties.
It is important to recognize that adsorptive separation processes on granular molecular sieves are two-step batch processes involving successive adsorption and desorption of molecules. In contrast, membrane separations are continuous processes that are accomplished by applying a driving force across the membrane (e.g., pressure gradient, concentration gradient, or temperature gradient). Thus, membrane separations do not require regeneration of the active sites in the membrane by desorption. Instead, a vapor-phase feed stream is continuously applied to one side of the membrane while purified product is continuously removed from another (permeate) side of the membrane. Because zeolite membranes allow continuous separation of multi-component mixtures, they offer significant advantages over zeolite granules, including less capital expenditure for equipment and fewer processing steps.
Despite the perceived advantages of zeolite membranes, their use in separations and catalysis poses significant challenges. Because their ability to separate molecules depends on surface properties and pore structure, which can vary significantly among different types of zeolites, many zeolite membranes demonstrate limited selectivity for separating mixtures of molecular components. Previous attempts to improve membrane performance have met with limited success. For example, post-synthesis treatments such as CVD modification or coke deposition may block access to the zeolite pore system and/or reduce pore entrance diameters, thereby decreasing flux through the membrane.
Although the acid hydrogen form of zeolite membranes is useful for catalytic membrane reactors, synthesis of acidic zeolite membranes is a complex process. Conventional synthesis of acid zeolite membrane requires the use of alkali metal hydroxides. Subsequent steps involve acid treatment or ion exchange with an ammonium salt solution, followed by thermal decomposition of the ammonium ion to obtain the acid hydrogen form of zeolite membranes.
The present invention overcomes, or at least mitigates, one or more of the problems set forth above.